In order to further examine spatial attention across a broad range of visual and auditory tasks, a neuroanatomical model is proposed, postulating dysfunction of interacting corticocortical circuits associated with different aspects of selective attention in DAT. A series of eight studies in patients with dementia of the Alzheimer's type (DAT), age-matched controls, and young, "young-old" and "old-old" adults will be conducted, with the following aims: (1) To examine the functional characteristics of spatial attention in mildly demented DAT patients across a broad range of visual, auditory, and cross-modal tasks; (2) To examine the neural mechanisms underlying spatial attention deficits in early DAT; (3) To examine age-related changes in these attentional functions, particularly in the old-old. For the second aim, positron emission tomography (PET) and event-related brain potentials (ERPs) will be used to disclose spatial and temporal properties, respectively, of spatial attention in the human brain. PET will aid in identifying affected brain regions in DAT. ERPs will be used to investigate the temporal locus of spatial attention changes in DAT.